kernel - usched_dfly revamp

kernel - usched_dfly revamp

* NOTE: This introduces a few regressions at high loads.  They've been
  identified and will be fixed in another iteration.

  We've identified an issue with weight2.  When weight2 successfully
  schedules a process pair on the same cpu it can lead to inefficiencies
  elsewhere in the scheduler related to user-mode and kernel-mode
  priority switching.  In this situation testing pgbench/postgres pairs
  (e.g. -j $ncpus -c $ncpus) we sometimes see some serious regressions on
  multi-socket machines, and other times see remarkably high performance.

* Fix a reported panic.

* Revamp the weights and algorithms signficantly.  Fix algorithmic errors
  and improve the accuracy of weight3.  Add weight4 which basically tells
  the scheduler to try harder to find a free cpu to schedule the lwp on
  when the current cpu is busy doing something else.